Why has my WPMU installation started trying to serve a subdomain site from the wrong directory, and

I was called out of the country for a few weeks after the Independence Day holiday and deferred further development on a subdomain site that had been working just fine. Technically, I was still working out how to make membership work for the site, but was in no hurry. Sometime in the interim, the subdomain site http://sub.domain.com/ has stopped serving correctly, resulting in that lovely “Index of…cgi-bin/” page and no connection to the wp-login (404 error). http://www.domain.com/ serves correctly, the database driving the Multisite installation appears to have all relevant tables, wildcard subdomains haven’t changed, and http://test.domain.com/ forwards to http://domain.com/wp-signup.php?new=test as expected.

But, very specifically, the Apache error log shows for the 21st of Jul (and later) – entries like:

[Sun Jul 21 21:38:14 2013] [error] [client 98.*.*.*] attempt to invoke directory as script: /home/subuser6/public_html/cgi-bin/, referer: http://sub.domain.com/

[Sun Jul 21 21:38:21 2013] [error] [client 98.*.*.*] File does not exist: /home/subuser6/public_html/login

My WordPress Multisite [subdomain] installation is in the following directory:

/home/admin/public_html

Any thoughts on how or where I can go to begin to put this right?